In cold climates particularly where occupied spaces must be heated, air in these spaces tends to have low relative humidity. This is uncomfortable and sometimes even unhealthy. To remedy this problem, people use humidifiers to add humidity to the air in these spaces.
Humidifiers have a variety of different designs. There are small stand-alone units intended for a single room. Larger units are designed for permanent installation as a component of a central heating system. These add moisture to the stream of heated air passing through the furnace duct to the occupied space. The latter type of humidifier will be referred to as an "in-duct" humidifier hereafter. The humidifier whose description follows is an improvement to one common type of in-duct humidifier.
There are a number of different designs for in-duct humidifiers. The kind which is involved here has an air-permeable pad, typically made from a number of similarly sized layers of thin expanded aluminum sheet stacked to a thickness of perhaps 1.5 in. (3.8 cm.). The layers of aluminum sheet are bonded to each other so as to create a pad structure having a rectangular box-like shape. The pad is placed in or near the furnace duct so that air warmed by the furnace can flow through it. Water is allowed to drip onto the top surface of the pad at a rate which keeps the pad moist from top to bottom. The warm air passing through the pad evaporates water in the pad, adding humidity to the air.
The water drips onto the pad from what is called a water distribution tray, or simply a tray. The tray extends along the top surface of the pad and has a reservoir for holding a small amount of water. Water is fed to the tray from the building water supply, and flow is controlled by a solenoid valve. The tray has holes spaced along its bottom through which water flowing into the tray falls onto the top of the pad. By properly selecting the rate at which water is added to the tray, the pad can be kept moist from top to bottom. The pad, the tray, and a housing supporting them in the proper spatial relationship comprise the most important elements of an in-duct humidifier. It is important for efficient operation that the tray evenly distribute water across the entire width of the pad.
There are water distribution trays now known which have a number of holes distributed along the length of the tray and use individual ducts or channels each for conducting water to each hole. The idea apparently is that using individual channels to conduct water to the holes allows each hole to receive a full measure of the water, thereby assuring that the pad is evenly and fully soaked throughout. These designs do not always fully realize these goals and indeed may sometimes also cause further problems. For example, problems may arise that still prevent complete and uniform saturation of the pad. This may happen if the tray is not perfectly level, preventing an adequate amount of water to flow to all parts of the pad's top surface.
I have found that it is also important for all of the water in the tray to promptly drain onto the pad when water flow stops. This eliminates undrained pools of water which will continue to dry, depositing the minerals dissolved in this pooled water on the tray surfaces. Over time, these mineral deposits can build up to a level which interferes with the operation of the tray itself. The use of a number of individual channels to supply water to individual holes tends to exacerbate this problem.